Field of the Invention
The present invention relates to a monopolar type ion exchange membrane electrolytic cell assembly.
Various types of electrolytic cells have been proposed as electrolytic cells for producing chlorine and alkali metal hydroxides wherein ion exchange membranes are used as diaphragms. In many cases, a filter press type electrolytic cell assembly is used in which a plurality of rectangular frames (compartment frames) are assembled and clamped.
Types of the electrolytic cells are generally classified based on the difference in the manner of electrical connection into bipolar electrolytic cells of series connection type and monopolar electrolytic cells of parallel connection type. The monopolar type electrolytic cells with which the present invention is concerned, have merits such that control of the current capacity is simple and conversion from a mercury method or an asbestos diaphragm method is easy. Accordingly, a number of monopolar type electrolytic cells have been practically developed.
Generally, an ion exchange membrane electrolytic cell is required to have a function of supplying sufficient electricity (electric current) to the anode and cathode and a necessary amount of electrolytes to conduct the electrode reaction certainly and, at the same time, allowing the ion exchange membranes to perform their own function to minimize the power consumption for electrolysis without damaging the ion exchange membranes. Accordingly, with respect to the construction of a monopolar type electrolytic cell, the method for supplying electricity to the cell and determination of the size of the electrolyzing area and the distance between the electrodes, etc. become important design factors.
With respect to the method for supplying electricity and the size of the electrolyzing area, the method for supplying electricity usually tends to be complicated as the size of the electrolyzing area is enlarged.
Namely, the single plate type monopolar cell disclosed in Japanese Unexamined Patent Publication No. 67879/1983 or Japanese Examined Patent Publication No. 39238/1987, has a simple structure, since the electrode plate itself serves as a power supply member and there is no other power supply means. However, such a structure can hardly be applied to a large scale electrolytic cell, since the loss due to resistance of the electrode plate increases as the electrolyzing area increases. Further, with a monopolar cell of the type reinforced by ribs, wherein electrodes are fixed to the ribs and/or the rods it is possible to freely adjust the electrolyzing area by arranging suitable power supply rods and/or power supply ribs, as shown in Japanese Examined Patent Publication No. 10956/1982 or Japanese Unexamined Patent Publication No. 210980/1982. However, in this case, it is essential to use power supply rods and/or ribs, and the structure is complex. Further, there was a substantial voltage loss accompanying the power supply through the ribs and/or the rods.
Reduction of the distance between electrodes, as an object of the present invention, is an important factor of the cell structure. The purpose of reducing the distance between the electrodes is to lower the voltage for an electrolysis. Namely, as the distance between the electrodes increases, the current path from the anode to the cathode increases, whereby the voltage loss resulting from the passage of current in the electrolyte will increase. Further, in the vicinity of electrodes, gas bubbles will be formed by the electrolysis, and such bubbles tend to increase the substantial electric resistance of the electrolyte, whereby the voltage loss will be further increased.
As another adverse effect of such bubbles, it is also known that the bubbles adhere to the surface of ion exchange membranes to shut out the current path whereby the cell voltage will be increased.
With respect to the adhesion of such bubbles to the membranes, it has been proposed to solve the problem by a method for preventing the adhesion of bubbles by bonding hydrophilic inorganic particles to the membrane surface, as shown in Japanese Examined Patent Publication No. 59185/1987.
It should ideally be possible to shorten the distance between the electrodes by preparing the anode and the cathode perfectly flat and putting them together with a membrane interposed therebetween. However, it is practically unavoidable that some irregularities or distortions are formed during the preparation of the electrode.
However, with respect to a single plate type monopolar cell having an electrolyzing area (portion) with a small width, reduction of the distance between the electrodes has been realized by putting together anode and cathode plates flattened under high dimensional precision with an ion exchange membrane interposed therebetween and clamping them by placing a thin gasket along the periphery of the electrolyzing area, as shown in Japanese Examined Patent Publication No. 37878/1985.
On the other hand, a complicated structure is required for a large size monopolar cell wherein electrodes are reinforced by ribs. As mentioned above, with a large size monopolar cell, it is practically impossible to finish the electrode surface to be completely flat, since various mechanical processings are required, and if the anode surface and the cathode surface are simply put together, there will be a portion where the electrodes abut strongly each other through the membranes, while there will be a portion where the distance between the electrodes is substantially enlarged. As a method for bringing the anode and the cathode in close contact with each other through the membranes while absorbing such a dimensional difference caused by such a lack in the precision for the preparation, it is known to support a flexible cathode or anode by a conductive spring member and to bring the flexible electrode in close contact with the facing electrode by means of the resiliency of the spring, as shown in Japanese Examined Patent Publication No. 3236/1987, or to deform flexible anode and cathode by means of conductive ribs arranged alternately to bring them in contact to each other, as shown in Japanese Examined Patent Publication No. 9192/1987.
Further, as disclosed in Japanese Examined Patent Publication No. 53272/1988 or Japanese Unexamined Patent Publication No. 163101/1983, a method is known wherein a resilient wire mat is provided between an ion exchange membrane and a flexible cathode, so that the cathode is brought in contact with the anode while ensuring the electric connection by the contact of the wire mat. Further, as disclosed in Japanese Unexamined Patent Publications No. 55006/1983 and No 55007/1983, a method is known wherein a current distributing member is divided into two sections and an electrode structure constituting an electrode is bent outwardly so that the electrode is brought in close contact with an ion exchange membrane by the restoring force of the electrode structure.
In these methods except for the case of the first mentioned single plate monopolar cell, a certain resilient member is required to press the electrode in order to bring the electrode in contact with a membrane, and the resilient member is required to have an electrically conductive function at the same time, whereby there has been the following problem. The resilient member is designed to be electrically connected with the electrode by a method such as bonding or contacting, but in order to impart an adequate conductive function, a resilient member having a large cross-sectional area for passage of the electric current or a pressing mechanism having a large contact area with a power supply member, is required. Consequently, a large pressure will be exerted to the pressing electrode.
The ion exchange membrane used as a diagram is a thin plastic film and is likely to be damaged when pressed with such a strong force from an electrode as mentioned above.
Also from the viewpoint of the preparation of an electrolytic cell, with respect to a large size electrolytic cell having a large current capacity and a large electrolytic area, a complicated system is required to accomplish uniform current supply and uniform pressing pressure simultaneously, and thus the preparation of such electrolytic cell has been difficult.
It is an object of the present invention to overcome the complexity of the conventional anode compartment assembly and cathode compartment assembly in a large size monopolar cell and, further to easily reduce the distance between the electrodes to bring the anode and the cathode close to or in contact with each other through the membrane without damaging the membrane.
Reduction of the distance between electrodes is an object of the present invention and an important factor of the cell structure. The purpose of reducing the distance between the electrodes is to lower the voltage for electrolysis. Namely, as the distance between the electrodes increases, the current path from the anode to the cathode increases, whereby the voltage loss resulting from passage of current in the electrolyte will increase.
The present invention provides a monopolar ion exchange membrane electrolytic cell assembly comprising a plurality of unit electrolytic cells connected electritically in parallel to one another, each formed by clamping an anode compartment frame and a cathode compartment frame with an ion exchange membrane interposed therebetween, the anode and cathode compartment frames each having a feeding and discharging system for an electrolyte and a discharging system for generated gas, wherein:
(a) an anode is made of a foraminous plate fixed to the anode compartment frame so that it is close to or in contact with the ion exchange membrane, and electricity is supplied to the foraminous plate via power supply rods and/or power supply ribs from a power source located outside the cell,
(b) a cathode is made of flexible foraminous metal plate having good conductivity with an electric resistance at 20.degree. C. of not higher than 10 .mu..OMEGA..cm so that the cathode itself has a current collecting function, and one peripheral end thereof is extended outward from the cell to conduct the electricity to the exterior of the cell, and, preferably,
(c) the flexible foraminous cathode plate is pressed by a resilient member from the side opposite to the side facing the ion exchange membrane, whereby the flexible cathode plate is deformed so that the cathode is close to or in contact with the ion exchange membrane.
Now, the present invention will be described in detail with reference to the preferred embodiments.